Reflections drawer: implement promote-to-knowledge-base action

[emp3thy]

Summary

The Reflections-drawer promote-to-knowledge-base action is currently deferred. Confirmed reflections feed memory.retrieve, but the markdown knowledge corpus (queried via knowledge.search / knowledge.list) is the durable, human-curated layer. Without a promote action, valuable lessons sit in per-project reflection state forever and never graduate to cross-project knowledge.

Originally noted as spec §8 (Reflections tab actions list: "promote to knowledge base") and §13 (Out of scope: "Promotion-to-knowledge-base workflow — can be added as a simple reflection action in a later phase"). Plan-side framing was in the now-deleted Phase 9 plan; the feature itself is still wanted.

Why this matters

User has flagged this as a key part of the system, not a nice-to-have. The reflection corpus accrues per-project; knowledge_base entries are durable and cross-project. Today there is no path from one to the other.

Implementation hooks already in place

• KnowledgeService has the write methods.
• reflection_sources joins reflections to observations for evidence trail.
• The drawer UI route is the natural call site.

Acceptance

• Drawer action: "Promote to knowledge base" on a confirmed reflection.
• Writes a KnowledgeDocument with the reflection body + evidence (linked observations).
• Reflection is marked promoted (or remains; decide UX).
• knowledge.search returns the promoted entry.
• Tests cover happy path + idempotency on re-promote.